No portion of the central nervous system, with the possible exception of the neocortex, has undergone as obvious a development in the course of phylogeny as the cerebellum. A variety of centers, such as the nucleus ddrsalis, red nucleus and others, undergo concurrent phylogenetic enlargement with the cerebellum. The evolutionary development and function of the cerebellum is related to modifications of those areas projecting to it and those areas receiving fibers from the cerebellum. This study will be conducted on the tree shrew (Tupaia), a generalized quadruped, the bushbaby (Galago), a vertical clinger and leaper, and the slow loris (Nycticebus) a specialized quadrupedal climber. These animals represent key levels in primate phylogeny and are the best extant examples of locomotor specialization evident in the fossil record of Eocene primates. Using these forms the cerebellar corticonuclear, corticovestibular and other corticofugal and spinocerebellar pathways will be studied using silver degeneration methods. Collateral projections to Clarke's nucleus and other nuclei of the spinal grey will be considered following removal of the dorsal root ganglia. Stereotaxic lesions will be placed in the deep cerebellar nuclei and the complete cerebellar efferent system characterized. These animals represent a relatively closely 'graded-series' on the phylogenetic scale, thereby giving this approach to cerebelluaar function and evoluation more validity than comparing divergent evolutionary forms such as the rat, cat and monkey. The results will be correlated with the phyletic level and locomotor style of each form and considered in light of current theories of locomotor evolution in primates.